Stoichiometric air fuel ratio: Definition, Formula, Calculation, Examples [with pdf]

What is stoichiometric air fuel ratio?

The stoichiometric air fuel ratio is the ratio that gives the amount of air required for the complete combustion of the unit amount of fuel.

This ratio is calculated on the molecular level. The stoichiometric ratio gives the exact amount of air required for the complete combustion of any fuel.

If the mixture has a lower air-fuel ratio than a stoichiometric ratio then it is known as a lean mixture.

And if the air-fuel ratio of the mixture is higher than the stoichiometric ratio then this mixture is considered a rich mixture.

Stoichiometric air fuel ratio for different fuels:

Here are the stoichiometric ratios for the different fuels:-

FuelsStoichiometric air-fuel ratio
Gasoline14.7:1
Octane15:1
Hydrogen34.5
Methane17
Methanol6.5:1
Diesel14.5:1
Ethanol9:1

How to calculate stoichiometric air fuel ratio?

Here are the steps to find the stoichiometric ratio of the fuel.

Step 1] Find the chemical equation of the oxidation of the fuel.

Step 2] Balance the equation.

Step 3] Find the molecular weight of fuel and the weight of the oxygen.

Step 4] Find the mass of oxygen: mass of fuel ratio

Step 5] Divide this value by 0.232 as there is only 23.2 percent of oxygen is present in atmospheric air. This is the value of stoichiometric air-fuel for the particular fuel.

Stoichiometric air fuel ratio example:

Stoichiometric air-fuel ratio for methane:-

The balanced chemical equation for the oxidation of methane is,

`CH_{4}+2O_{2}\rightarrow CO_{2}+2H_{2}O`

Atomic weight of C = 12.01
Atomic weight of H = 1.00
Atomic weight of O = 16

∴ Molecular weight of CH4 = 12.01 + 4(1) = 16.01

Molecular weight of `O_{2}` = 2(16) = 32

The oxygen:fuel ratio is,

=`\frac{\text{Weight of oxygen}}{\text{Weight of methane}}`

=`\frac{\text{Molecular Weight of}\ 2O_{2}}{\text{Molecular Weight of methane}\ CH_{4}}`

`=\frac{2\times 32}{16.01}`

= 3.997 %

The stoichiometric air fuel ratio is given by,

`=\frac{3.997}{0.232}`

= 17.2

Hence the stoichiometric air-fuel ratio for methane is 17.2:1.

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